Water Column Stratification

The development and persistence of hypoxia results from two principle factors. First, stratification of the water column must occur to prevent mixing of diffused oxygen from the surface (Rabalais et al. 2002). The degree of stratification within a body of water depends on salinity, temperature, and circulation (Reshetiloff 1995). Surface warming and freshwater input from rivers leads to the development of a top layer consisting of warmer less saline waters. This layer resides atop cooler, saltier, thus denser water masses. A strong pycnocline, or area of intense mixing between the two layers, maintains separation between surface and bottom waters, restricting the ability of oxygenated surface waters to mix with the bottom layer (Reshetiloff 1995). Temperature and salinity are both significant factors in the stratification of both the Chesapeake Bay and the Gulf of Mexico (Figure 1) (Rabalais et al. 2002).

Figure 1. Stratification intensities in the Chesapeake Bay. Source: EPA

Eutrophication

Another important cause influencing the occurrence of hypoxia is the decomposition of organic matter in bottom waters. In both the Gulf of Mexico and Chesapeake Bay, the primary source of organic matter is phytoplankton blooms that result from increased nutrient inputs (Rabalais et al. 2002). Two phytoplankton groups are typically responsible for eutrophication. Cyanobacteria (Cyanophyceae, or blue-green algae) largely compose algal blooms in freshwater systems, while dinoflagellates (Dinophyceae) are to blame in marine systems (Paerl 1988). Both groups can be found in the Chesapeake Bay and the Gulf of Mexico, as each contains both fresh and saline waters (Breitburg 1990; Rabalais et al. 1999). Phytoplankton that are not incorporated into the food web by primary consumers sink to bottom waters, where they are decomposed aerobically by bacteria (Rabalais et al. 2002). The increased consumption of oxygen resulting from the decomposition of large quantities of organic material results in oxygen depletion, or hypoxia. When a layered water column is present, hypoxia persists because stratification hinders mixing between hypoxic bottom waters and oxygen-rich surface waters.

Figure 2. The processes leading to hypoxia. Source: Environmental Health Perspectives

Figure 3. Nutrient sources in a watershed. Source: Environmental Health Perspectives

Sources of Nutrients

The three main limiting factors for phytoplankton growth in the Chesapeake and Gulf of Mexico are primarily nitrogen, and also phosphorous and silica. A number of human activities have resulted in the increased input of these nutrients (Figure 3). The chief source of nutrients in the Gulf of Mexico is nutrient-rich agricultural runoff from the Mississippi and Achatfalaya Rivers, as much of the land incorporated by the Mississippi River drainage basin is farmed extensively (Rabalais et al. 2002). An estimated 11.6 million metric tons of nitrogen is deposited into the Gulf of Mexico by the Mississippi River, and approximately 80% of this nitrogen is a result of non-point runoff of fertilizers and animal manure applied to agricultural fields (Goolsby et al. 1997) (Figure 4). In addition, nitrogen from fixed legumes, human domestic waste, rainfall deposition, and municipal and industrial point discharges are also sources of anthropogenic nitrogen input from the Mississippi River (Goolsby et al. 1997). In the Chesapeake Bay, 28% of nitrogen input results from nitrogen fixation in agricultural lands, and 15% is due to fertilizer application (Hagy et al. 2004).

In addition to agricultural runoff, a large portion of nutrient input to North Atlantic coastal waters, including the Chesapeake Bay, also comes from deposition of nitrogen from the combustion of fossil fuels (Rabalais et al. 2002). Atmospheric deposition accounts for 27% of nitrogen influx to the Chesapeake Bay (Hagy et al. 2004). Wet and dry fallout, and also direct groundwater discharge through shallow coastal aquifers is a significant and often overlooked source of nutrients influencing primary production and eutrophication dynamics in coastal ecosystems (Perl 1997). Groundwater discharge is most likely an insignificant source of nitrogen in the Gulf of Mexico, as few shallow aquifers exist along the Louisiana coast.

Figure 4. Quantity of Nitrogen in commercial fertilizer applied to fields by geographic region. Source: USGS